It is the state of the art, for example, in accordance with DE 33 39 717 A1, to achieve the cooling of an internal combustion engine by means of evaporation of a cooling agent. The temperature of a component on the combustion chamber side can be recorded by means of a sensor, and the vapor pressure can be regulated as a function thereof. In this way, an adjustment of the cooling of the internal combustion engine to changing operating conditions is possible within a limited scope. If the internal combustion engine becomes, for example, only a little loaded, then a higher component temperature is suitable, which can be adjusted by increasing the vapor pressure.
An efficient evaporation cooling is then particularly possible when a specific vapor state is present. This is, in practice, a state within the wet vapor area. For example, wet vapor with a residual moisture of around 5 percent may be optimal (x=0.95). That is, overheating of the vapor should be avoided. Overheating primarily results in a low heat transfer, which would make the cooling of an engine less economical. This is due to such a procedure (with overheating) resulting in high wall temperatures as well as unfavorable thermal gradients in the range of final boiling point and start of overheating. Excessive component loads and potential damage can therefore not be excluded. In order to influence or adjust the desired vapor state, it is necessary to regularly provide, in any case, for the delivery of a given amount of coolant in a wide range of varying dissipating heat with regard to the operation of an engine. In summary, the knowledge of the instantaneous vapor state is essential for an optimal evaporation cooling of an engine. With pressure and temperature sensors, this state in the wet vapor area cannot, or cannot satisfactorily, be determined. That is, it is thus not possible to unequivocally determine which state the cooling fluid is in, i.e., whether it is close to the liquid state or near the gaseous state. It would be conceivable to implement a slight overheating at the outlet, i.e., downstream of the engine to be cooled, so that the energetic state can always be determined explicitly. However, as stated, overheating is detrimental to economical and safe cooling.